1. Field of the Invention
The present invention relates to a set of golf clubs, and more particularly, to an improvement of club-heads of a set of golf clubs.
2. Description of the Related Art
Generally, the direction of flight of a ball is determined by the orbit of a swing of a club-head just before the impact of the club-head with the ball and the direction in which the club-head is facing at the time of impact of the club-head with the ball. If the orbit of the swing of the club-head is not in alignment with an objective direction of flight of the ball just before the impact of the club-head with the ball, the direction of flight of the ball deviates from the objective direction. In turn, if the club-head is facing toward the left or right of the objective direction of flight of the ball at the time of impact of the club-head with the ball, the flight of the ball will be toward the left or right of the objective direction of flight of the ball.
As mentioned above, the direction of flight of the ball is influenced by the direction in which the club-head is facing or the orbit of the swing of the club-head, and this influence given by the direction in which the club-head is facing or the orbit of the swing of the club-head to the direction of flight of the ball is varied by the length of time the club-head is in contact with the ball. That is, when the contact time is short, the direction of flight of the ball only slightly affected by the swing orbit of the club-head, and a much greater affect is given by the direction in which the club-head is facing. Conversely, when the contact time is longer, the direction of flight of the ball is only slightly affected by the direction in which the club-head is facing, and a much greater affect is given by the swing orbit of the club-head.
Generally, golf players try to keep the face of the club-head perpendicular to the objective direction of flight of the ball when the ball is hit by the club-head, and to ensure that the swing orbit of the club-head conforms with the objective direction of flight of the ball just before the impact of the club-head with the ball. In this case, it is very difficult to control the direction in which the club-head is facing during the impact of the club-head with the ball, because the impact is effected in a very short time, but it is relatively easy to control the swing orbit of the club-head because the swing action of the golf club is effected in a relatively long time. Therefore, it is desirable to correct the swing orbit of the club-head in order to improve the direction of flight of the ball.
When using a conventional set of golf clubs, it has been shown that, generally, a mishit and a subsequent deviation of the direction of flight of the ball tend to occur more often with an increase in the length of the golf club used, because the club-shaft is twisted further and thus the club-head is more apt to face toward the left or right of the objective direction of flight of the ball at the time of the impact of the club-head with the ball.
Japanese Unexamined Utility Model Publication (Kokai) No. 59-193462 discloses a set of iron golf clubs in which the thicknesses of the face portions or ball-hitting portions of the club-heads are increased gradually in accordance with an increase of the club-lengths. In such a set of iron golf clubs, times that the club-heads are in contact with the ball during the impact therebetween are decreased in accordance with an increase of the thicknesses of the face portions of the club-heads, and it becomes more difficult to control the direction in which the club-head is facing in accordance with decrease in the time that the club-head is in contact with the ball, as described above. Therefore, such a set of iron golf clubs has a disadvantage in that the direction of flight of the ball is more apt to be deviated from the objective direction of flight of the ball in accordance with the increase of the club-length.